91 454 TBI

[Yenko427]

I have a 91 R3500 with a gen v 454. I have a ALDL adapter and was watching my values when pulling my camper. When I put the truck in 4 gear and the lock up converter kicks in I notice that my timing goes retard and my Map sensor values go into the red and this is with over 50 percent throttle. If I let off of the throttle below 50 percent the Map sensor values go back up and the timing goes back advanced. If if keep the truck in 3 gear it never gets more then 40 percent throttle so it doesn’t go into retard in timing.

[dave w]

The 91 R3500 4.4 / 4L80E will use computer 16147060: http://www.gearhead-efi.com/Fuel-Inj...Information-85

Usually 454 timing tables will reduce timing at higher RPM's. Perhaps the timing is reduced for spark knock?

A data log using TunerPro RT will show knock retard.

dave w

[1project2many]

How heavy is the truck and camper combination? The TBI 454 is not known for power.
Is this a problem you've been chasing for a while? Is the truck new to you?
Do you know the final drive ratio?
What area or altitude are you driving at?
Are you asking because you've just bought a new tool and you've noticed these values?

[Yenko427]

How heavy is the truck and camper combination? The TBI 454 is not known for power.
Is this a problem you've been chasing for a while? Is the truck new to you?
Do you know the final drive ratio?
What area or altitude are you driving at?
Are you asking because you've just bought a new tool and you've noticed these values?

The truck is R3500 dually that weighs about 10,000 pounds. I’ve been chasing this demon for a while. First I cut off the cat because it was clogged. And I’ve replaced all the sensors. I did just purchase the adapter to watch my live data. The camper I’m pulling weighs 10,000 pounds too. The axle ratio is 373 with a 4L80E transmission. Just seems odd that if I push the accelerator past the 50 percent it makes the timing go retard and the MAP sensor goes negative.

[1project2many]

That's a pretty big combination for the old 7.4. I'm not surprised the MAP goes high in overdrive at 50% throttle. I am surprised the transmission isn't downshifting. Retarded or negative timing can be caused by knock due to insufficient fuel quantity or quality, Or the tool you're using may not be displaying spark values correctly.

Watch the oxygen sensor to see if it's showing .750 to .800mV or more under wide open throttle. Lower values suggest fuel pressure and volume should be checked.